Pfizer and Allergan are joining in the biggest buyout of the year, a $160 billion stock deal that will create the world’s largest drugmaker.

It’s also the largest so-called inversion, where an American corporation combines with a company headquartered in a country with a lower corporate tax rate, saving potentially millions each year in U.S. taxes.

President Barack Obama and Treasury Secretary Jack Lew have both spoken out against inversions and introduced new rules last year to make them harder.

New Pfizer expects tax rate of 17-18% in first year after closing. Down from 25% this year $PFE $AGN

— Michael de la Merced (@m_delamerced) November 23, 2015

The deal is valued at $363.63 per Allergan share. Allergan shareholders will receive 11.3 shares of the combined company for each of their shares, while Pfizer stockholders will get one share of the combined company.

Pfizer, which makes the cholesterol fighter Lipitor, will keep its global operational headquarters in New York. But the drugmaker will combine with Botox-maker Allergan as a company that will be renamed Pfizer Plc and will have its legal domicile and principal executive offices in Ireland.

Several U.S. drugmakers have performed inversions through acquisitions in the past several years, in part to escape high U.S. corporate tax rates. The list of companies includes Allergan, which still runs much of its operation out of New Jersey, and the generic drugmaker Mylan.

U.S. efforts to curb the practice have so far proven ineffectual.

Last year, Pfizer unsuccessfully tried to buy British drugmaker AstraZenenca in a roughly $118 billion deal that would have involved an inversion. Those talks eventually collapsed when the two sides couldn’t agree on a price.